Why is My Radiator Not Heating Up? Common Causes & Fixes
A radiator that stays cold (or only warms in patches) is a common and usually fixable problem. Here are the most likely causes and what to do about each.
Cold at the top — trapped air
If a radiator is hot at the bottom but cold at the top, trapped air is the usual cause. Bleeding the radiator releases the air: turn off the heating, use a radiator key on the bleed valve until water appears, then close it. A working bleed vent makes this quick and mess-free.
Cold at the bottom — sludge
If a radiator is warm at the top but cold along the bottom, a build-up of sludge (magnetite) is likely restricting flow. This usually needs a power flush or chemical clean by a heating engineer.
Whole radiator cold — stuck TRV pin
If a single radiator is completely cold, the pin under the TRV head may be stuck down. Remove the head and gently free the pin (it should spring up and down). Check the lockshield valve at the other end is open too.
Some radiators cold — balancing
If radiators furthest from the boiler stay cool while nearer ones get hot, the system may need balancing — adjusting the lockshield valves so hot water is shared evenly around the system.
When to call a professional
If bleeding, freeing the pin and checking the valves doesn't help, or if you suspect sludge or a pump/boiler fault, it's worth calling a qualified heating engineer.
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